A relevant description of the background for the present disclosure is found in EP 2026601 A1 from which most of the following is taken.
People who suffer from a hearing loss most often have problems detecting high frequencies in sound signals. This is a major problem since high frequencies in sound signals are known to offer advantages with respect to spatial hearing such as the ability to identify the location or origin of a detected sound (“sound localisation”). Consequently, spatial hearing is very important for people's ability to perceive sound and to interact with and navigate in their surroundings. This is especially true for more complex listening situations such as cocktail parties, in which spatial hearing can allow people to perceptually separate different sound sources from each other, thereby leading to better speech intelligibility [Bronkhorst, 2000].
From the psychoacoustic literature it is apparent that, apart from interaural temporal and level differences (abbreviated ITD and ILD, respectively), sound localisation is mediated by monaural spectral cues, i.e. peaks and notches that usually occur at frequencies above 3 kHz [Middlebrooks and Green, 1991], [Wightman and Kistler, 1997]. Since hearing-impaired subjects are usually compromised in their ability to detect frequencies higher than 3 kHz, they suffer from reduced spatial hearing abilities.
Frequency transposition has been used to modify selected spectral components of an audio signal to improve a user's perception of the audio signal. In principle, the term “frequency transposition” can imply a number of different approaches to altering the spectrum of a signal. For instance, “frequency compression” refers to compressing a (wider) source frequency region into a narrower target frequency region, e.g. by discarding every n-th frequency analysis band and “pushing” the remaining bands together in the frequency domain. “Frequency lowering” refers to shifting a high-frequency source region into a lower-frequency target region without discarding any spectral information contained in the shifted high-frequency band. Rather, the higher frequencies that are transposed either replace the lower frequencies completely or they are mixed with them. In principle, both types of approaches can be performed on all or only some frequencies of a given input spectrum. In the context of this invention, both approaches are intended to transpose higher frequencies downwards, either by frequency compression or frequency lowering. Generally speaking, however, there may be one or more high-frequency source bands that are transposed downwards into one or more low-frequency target bands, and there may also be other, even lower lying frequency bands remaining unaffected by the transposition.
Patent application EP 1742509 relates to eliminating acoustical feedback and noise by synthesizing an audio input signal of a hearing device. Even though this method utilises frequency transposition, the purpose of frequency transposition in this prior art method is to eliminate acoustical feedback and noise in hearing aids and not to improve spatial hearing abilities.